GPS is a satellite-based radio navigation system. The GPS system is divided into three segments: space, control, and user. The space segment comprises currently a constellation of 24 GPS satellites. The control segment comprises ground stations around the world that are responsible for monitoring the GPS satellite orbits, synchronizing the satellites' onboard atomic clocks, and uploading data for transmission or broadcast by the satellites. The user segment consists of GPS receivers used for both military and civilian applications.
Each GPS satellite, also called space vehicle (SV), broadcasts time-tagged ranging signals and navigation data. A SV essentially provides its signal transmit time and ephemeris to GPS receivers. A GPS receiver extracts the signal transmit time from its code tracking loop and compares it with the signal reception time determined by the receiver clock. Satellite clocks are synchronized with the GPS time, while the receiver clock, which does need to have good short term stability, is not. A difference between the receiver clock time and GPS time is called a receiver clock bias.
A time difference, between the signal transmit time and the signal reception time, is an apparent transit time of the signal from the satellite to the receiver. A pseudorange is the measured apparent transit time multiplied by the speed of light in a vacuum, which needs to account for the receiver clock bias, ionospheric delay, and other measurement corrections. If corrected pseudorange measurements from at least four satellites in view are available at a single measurement epoch (period or time interval, typically every one second), a receiver three-dimensional position and clock bias can be determined. Typically, the GPS receiver is configured to compute a delta position with respect to a previously obtained position and then update the position solution for the current epoch.
In addition to the ionospheric delay, the GPS ranging signals transmitted from a satellite to a receiver are subject to a variety of other noise and error sources, either intentionally or unintentionally, such as ephemeris data error, multipath, and jamming. Reflection is one type of multipath, where a GPS receiver only tracks a reflected signal while its direct signal is blocked, for example, by a building. Signal ranging errors are eventually turned into a GPS positioning error. In some urban canyon environments, multipath and reflections can compound to become a severe problem to GPS navigational systems.
In order to achieve a certain high level of position accuracy and integrity, GPS receivers usually implement a failure detection and exclusion (FDE) system to detect range measurement failures as quickly as possible within a relatively small probability of false detection. If a failed measurement is detected, it will not be used in the computation of the GPS position, velocity and time (PVT) determination at the current epoch.
The pseudorange measurement residual test is widely used in FDE systems or units of GPS receivers to detect pseudorange measurement failures. A pseudorange measurement residual is a difference between a corrected pseudorange measurement and a predicted range from the satellite to the receiver. The residual test computes the residuals and then compares them with a predetermined residual threshold. If the magnitude of a residual is larger than the threshold value, then the corresponding pseudorange measurement is detected as a failure; otherwise, the measurement passes the test. Only those that pass the residual test is used in the PVT determination. The tighter the threshold, the more effective the test is in detecting failures; however, a too tight threshold can result in an unfavorable increase in false detection probability. Therefore, the threshold should be properly valued. There are basically two types of pseudorange residual tests, i.e., pre-update and post-update pseudorange residual tests.
The post-update pseudorange residual test first updates the receiver position using all measurements, and then calculates the residuals based on the updated receiver position. If the measurements do contain some failures that are to be detected, then both the updated position and the calculated residuals are erroneous due to utilizing the failed measurements. Meanwhile, if there are four or fewer pseudorange measurements available, the system of equations, which is typically solved by a least-squares method to determine the three-dimensional receiver position and time, is not over-determined. Thus, the residuals based on the newly updated position are zeros theoretically. Therefore, this test is incapable to detect measurement failures when there are fewer than five satellites in view, which is generally the case in severe urban canyon environments.
The pre-update pseudorange residual test calculates residuals based on a previously obtained receiver position, detects and excludes failures, and then updates the position by utilizing good measurements only. Therefore, unlike in the post-update residual test, this updated position should not be jeopardized by failed measurements. The test is very useful if the pre-update receiver position is quite close to the current true position, and can still be functional even in cases when only four or fewer satellites are in view. Since possible receiver movement and clock frequency drift during one epoch can introduce a bias to the calculated residuals, typically GPS receivers employ some approaches to remove this residual bias. These approaches, however, utilize all available measurements, including potentially erroneous measurements, which reduce the effectiveness of the residual test.
Accordingly, there is a need for addressing the problems noted above and others previously experienced.
Illustrative and exemplary embodiments of the invention are described in further detail below with reference to and in conjunction with the figures.